This invention relates primarily to electronic projection of images and more particularly to techniques and equipment for processing sub-images prior to their projection onto a screen or other surface for the purpose of superimposing one or more sub-images onto one or more other sub-images.
U.S. Pat. No. 5,490,009 to Venkateswar, et al., incorporated herein in its entirety by this reference, discusses a display system in which two sub-images generated using spatial light modulators (SLMs) are partially superposed (with a spatial offset) and displayed simultaneously. As described in the Venkateswar patent, the system purportedly provides enhanced resolution of a projected image by merely increasing the number of SLMs. Either horizontal or vertical resolution, or both, can be enhanced. For example, to approximate a twofold increase in horizontal resolution, the system uses two SLMs, each having x pixels per row and y rows. Each of the two SLMs provides a xe2x80x9csub-image,xe2x80x9d generated from alternating samples of data, with the sampling rate providing 2(x) samples per row. The two images are partially superposed, with a horizontal offset of xc2xd the center-to-center horizontal spacing of the pixels of the SLM. The resulting image has a perceived quality approaching that of a system that generates a single image from a SLM having 2(x) pixels per row.
See Venkateswar, col. 2, ll. 32-43.
FIG. 1 of this application illustrates exemplary spatial superimposition of two imagers; one offset one-half pixel diagonally from the other. Stated differently, the display of FIG. 1 may be made by offsetting the pixels of Imager A (as compared to those of Imager B) by one-half their size in both the horizontal and vertical dimensions. To permit this type of superimposition, a base image xe2x80x9cIxe2x80x9d must be processed so as to divide it into two diagonally-offset components, sub-images xe2x80x9cIAxe2x80x9d (associated with Imager A) and xe2x80x9cIBxe2x80x9d (associated with Imager B).
Shown in FIG. 2 of the Venkateswar patent is a block diagram of a system for displaying superimposed sub-images. Although the illustrated system includes a block designated xe2x80x9cprocessor systemxe2x80x9d (element 23 of the drawing), the sole recitation of its function is as follows:
Processor system 23 prepares the data for display by performing various pixel data processing tasks. Processor system 23 includes whatever processing memory is useful for such tasks, such as field and line buffers. The tasks performed by processor system 23 may include linearization (to compensate for gamma correction), colorspace conversion, and line generation.
See id. col. 4, ll. 36-42. Thus, absent from the Venkateswar patent is any disclosure of specific techniques for processing base images to obtain any resolution-enhancing benefits of superimposition.
The present invention addresses such techniques for pre-processing of images to be superimposed for display. In general, the techniques, which utilize signal-processing algorithms, create from a higher resolution image a selected number of components, which components subsequently will be combined through superimposition of multiple imagers. The ultimate result of using these techniques is projection of images with higher resolution than that obtainable through use of each single imager.
Superimposition may be achieved within a projector by combining multiple imagers before the composite image is passed to the projection lens. Alternatively, superimposition may occur by overlaying multiple precisely-aligned projectors. Those skilled in the art will understand that techniques of the present invention are useful in either situation, however, and may be applicable in many cases without regard to the types of SLMs and light valves, sources, or engines employed. In particular, any SLMs utilized in the projection system mayxe2x80x94but need not bexe2x80x94the deformable mirror devices (DMDs) referenced in the Venkateswar patent.
Generally speaking, any display based on imagers with a discrete pixel array where the number of pixels for the horizontal and vertical dimensions of the image are fixed has a certain maximum spatial frequency that it can display. It is convenient to convert the spatial frequency of the display from pixels per unit area to a more common imaging dimension such as lines per millimeter which can be done by assuming a reference image size and dividing the number of pixels by that size in millimeters. When such displays are used to show images that are derived from photographic film, for example, such a relationship is particularly useful as it helps to describe how well a photographic image with a certain modulation transfer function (MTF) will be reproduced by the display. MTF is commonly expressed as a graph showing depth of modulation in percent versus lines per millimeter.
For a given reference image size, doubling the number of pixels of the imager will double the maximum spatial frequency that the display can represent. Unfortunately, such a doubling in each direction increases the number of pixels by four (2xc3x972) times, and the difficulty of fabrication of pixel array imagers such as DMDs is greatly increased by such large increases in pixel count.
It is also essential that for a given pixel count, the spatial frequency content of image data to be displayed must be limited so that the spatial frequency capability of the display is not exceeded. If this is not done higher frequencies will be incorrectly displayed as lower ones; a phenomenon commonly called aliasing.
By using the technique of superimposition one can simulate the effect of doubling the number of imager pixels. This requires that a starting image have at least the spatial frequency content from sampling that is two times the spatial frequency limit of either of the two rasters that will be superimposed. This increased spatial frequency content can then be subdivided between the two rasters using suitable re-sampling techniques.
Because the resulting display is composed of superimposed pixels of twice the size (offset by xc2xd pixel diagonally), there is a summation that takes place at the projection screen. This summation reduces the MTF available above the spatial frequency limit of a single imager, so that the MTF of a display with two superimposed imagers is not exactly equal to that of a single imager with twice as many pixels, but the improvement is still significant. The summation has the additional benefit that the visibility of subraster pitch artifacts in the display (such as the gaps between the mirrors of a DMD device) is reduced by the summation of two offset rasters. Additionally, it is necessary to take into account the fact that pixels are adding together at the display by compensating for the increased black level and the effective stretching of the dynamic range that will occur.